1. Field of the Invention
The present invention relates to a method and apparatus for erecting and removing offshore structures. The present invention is deemed particularly useful for use in deep and/or rough ocean.
2. Description of Prior Art
Derrick barges or crane ships have been extensively used for erecting and removing offshore structures. However, their lifting capacity is limited by their weight lifting capacity, water depth, environmental conditions, etc. Increasing the capacity of a derrick barge requires building larger, more powerful and versatile derrick barges, which can become prohibitively expensive. Even then, a larger barge would not provide the flexibility needed for removing and erecting different types of offshore structures currently in use for oil and gas exploration around the globe.
Recently, semi-submersible systems, utilizing floating tanks, have been designed that are towed to an offshore construction site for use in erecting or removing offshore structures. U.S. Pat. No. 4,683,832 is an example of such a structure. These systems, however, have many disadvantages: they afford little or no system flexibility; they address only the removal of part of an offshore structure; they are limited in their application by water depth; and they are unstable for use in deep and/or rough seas.
The present invention has solved major disadvantages of apparatus currently in use by providing a flexible system that is less expensive, very stable over a broad range of operating conditions, and extremely safe to operate especially in rough and/or deep sea operations.